This study was supported financially by the General Research Fund of the Research Grants Council, the Hong Kong Special Administrative Region, China (HKU 702707P and HKU 703109P), China Geological Survey, Guangdong Geological Survey and "Sustainable Water Environment" Strategic Research Sub-Theme in HKU. We thank Bob Ingleton, Xingxing Kuang, Haipeng Guo, Kouping Chen, Ming Tang, Shubing Wang, Kanghong Ai, Pu Yao, and students from the research group headed by Jiaoyao Chen in Zhongshan University for assistance with field drilling and sampling. Yongqiang Zong and Guangqing Huang are thanked for sharing their knowledge on Quaternary geology.